Describe the mechanism of mammalian breathing.

The process of breathing is made up of two components:

• Inhalation
• Exhalation

Inhalation is the process of filling the lungs with air. As they fill they expand and so the rib cage moves upwards and outwards.

Exhalation is the process where the lungs empty of air. As they empty they shrink back and the ribs move downwards and inwards.

Inhalation and exhalation are made possible by pressure changes within the thorax. There are three structures involved in these pressure changes:

• The diaphragm
• The internal intercostal muscles (expiration)
• The external intercostal muscles (inspiration)

The diaphragm is a sheet of muscle separating the abdominal cavity from the thoracic cavity. During inhalation, the diaphram contracts and flattens creating a larger volume in the thorax. As the volume increases, the pressure drops, this means that the pressure outside the lungs is greater than the pressure inside. thus air moves into the lungs. During exhalation the diaphragm relaxes and goes back to its dome shape, thus the thoracic space is smaller and the air pressure is greater inside the lungs than out, thus the air moves out.

The method of measuring these is by calculating the pulonary ventilation rate. This is the total volume of air moved into the lungs in one minute using the formula 'Pulmonary Ventilation Rate = Tidal Volume x Ventilation' where: Tidal Volume = the volume of air normally taken in for each breath when the body is at rest Ventilation Rate = the number of breathes taken in one minute (normally 12-20)